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Condensed Statements of Income and Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,184,423 | $ 1,285,782 | $ 1,060,595 | $ 931,604 | $ 1,081,144 | $ 1,173,518 | $ 967,962 | $ 852,807 | $ 4,462,404 | $ 4,075,431 | $ 4,050,492
Operating expenses | 1,037,306 | 1,135,909 | 969,217 | 827,073 | 1,190,673 | 1,207,553 | 876,248 | 781,750 | 3,969,505 | 4,056,224 | 3,644,125
Operating income | 147,117 | 149,873 | 91,378 | 104,531 | (109,529) | (34,035) | 91,714 | 71,057 | 492,899 | 19,207 | 406,367
Other income |  |  |  |  |  |  |  |  | 6,768 | 6,642 | 6,520
Interest expense |  |  |  |  |  |  |  |  | 83,917 | 76,699 | 81,354
Income (loss) before income taxes |  |  |  |  |  |  |  |  | 415,618 | (45,467) | 336,226
Income taxes |  |  |  |  |  |  |  |  | 136,736 | (31,146) | 110,274
Net income (loss) attributable to parent | 91,450 | 84,456 | 46,512 | 56,515 | (60,986) | (29,671) | 54,113 | 35,790 |  |  |
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 278,248 | (1,439) | 212,341
Comprehensive income (loss) |  |  |  |  |  |  |  |  | 289,449 | (2,474) | 197,286
MDU Resources Group, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 549,239 | 472,302 | 518,268
Operating expenses |  |  |  |  |  |  |  |  | 473,917 | 405,095 | 450,579
Operating income |  |  |  |  |  |  |  |  | 75,322 | 67,207 | 67,689
Other income |  |  |  |  |  |  |  |  | 3,709 | 3,925 | 2,710
Interest expense |  |  |  |  |  |  |  |  | 17,386 | 17,297 | 18,660
Income (loss) before income taxes |  |  |  |  |  |  |  |  | 61,645 | 53,835 | 51,739
Income taxes |  |  |  |  |  |  |  |  | 13,520 | 11,798 | 10,476
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 230,808 | (42,791) | 171,763
Net income (loss) attributable to parent |  |  |  |  |  |  |  |  | 278,933 | (754) | 213,026
Dividends declared on preferred stocks |  |  |  |  |  |  |  |  | 685 | 685 | 685
Earnings (loss) on common stock |  |  |  |  |  |  |  |  | 278,248 | (1,439) | 212,341
Comprehensive income (loss) |  |  |  |  |  |  |  |  | $ 289,449 | $ (2,474) | $ 197,286

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